According to below-described Non-Patent Documents, etc., it is known that, if the shortest distance (OCT: Outer Cladding Thickness) between a core center of a core (outermost core) positioned in an outermost and a cladding surface (coating) is short in the cross section perpendicular to a fiber axis of an optical fiber, particularly, a multi-core optical fiber (MCF), the leakage loss (LL) [dB/km] due to coupling of core-mode light from the core to the coating with a high refractive index.
In order to increase the utilization efficiency of the cross section of MCF, more cores are required to be packed. As an idea, it is conceivable to reduce the refractive index of the coating in order to suppress LL while OCT is reduced. In that case, a cladding mode is not sufficiently removed in the coating, and multi path interference (MPI), etc. due to re-coupling from the cladding mode to a core mode are easily generated. As a result, the signal quality of signal light, which propagates through the core, is deteriorated. In consideration of these, the coating is required to have a high refractive index at a certain level in order to achieve a predetermined MPI value and have a certain level of OCT in order to achieve a predetermined LL.
It has been described that the OCT required in core designing is 30 μm or more in Non-Patent Document 1 and is approximately more than 40 μm in Non-Patent Documents 2 and 3, and the required OCT is individually different depending on core structures and other structures. The level of LL has to be actually checked, and the measurement method thereof has been an indirect method which utilizes the excess loss (Excess Loss) shown in Non-Patent Documents 2 and 3.
In the left side of FIG. 1A, an example of MCF (cross-sectional structure of a MCF 10 orthogonal to a fiber central axis AX) is shown. The MCF 10 has a common cladding 4 comprised of silica-glass, and outermost cores (peripheral cores) 1 are circularly arranged at equal intervals about a central axis of a central axis of the common cladding 4, which is a fiber central axis AX. On an outer periphery of the common cladding 4, a coating 5 of a high-refractive-index resin is provided. In the example of FIG. 1A, as an option, as well as the structure disclosed in Non-Patent Document 2, an internal cladding 2, which has the same refractive index as that of the common cladding 4, and a trench layer 3, which has a refractive index lower than that of the common cladding 4 at this point, are sequentially provided on an outer periphery of the outermost core 1. FIG. 2 shows the relations of refractive-index distributions of the outermost core 1 and the coating 5, specifically, the refractive-index distributions of peripheral regions of the outermost core 1 and the coating 5 corresponding to the configuration shown in FIG. 1A. The outermost core 1 and the coating 5 are separated from each other via the common cladding 4, and, particularly, the outermost core 1 and the common cladding 4 are separated from each other via the internal cladding 2 and the trench layer 3. As can be seen from FIG. 2, the refractive index of the coating is a high refractive index compared with the refractive index of the outermost core 1; and, when part of the light subjected to wave-guiding through the outermost core 1 (core-mode light) reaches the coating 5, the reached light is coupled to the coating 5.
FIG. 3 is a graph showing relations between OCT of MCF and leakage losses LL. A vertical axis of FIG. 3 is shown by logarithmic display. It is known that increases in transmission losses of the outermost cores due to LL are exponentially increased when OCT is reduced. In order to suppress deterioration of signal-to-noise ratios (S/N ratios) due to transmission loss deterioration as much as possible, it is desired to reduce the leakage loss to an ignorable level. Non-Patent Documents 1 to 3 explicitly or implicitly state that, at a wavelength of 1625 nm, LL of the outermost core be suppressed to 0.001 dB/km.